There are some applications where a polyethylene or similar bag must be inflated and maintained at a pressure within a predetermined range. One particular application for such a bag is in the treatment of open skin wounds. As discussed in U.S. Pat. No. 5,478,310 of Evelyn Dyson-Cantwell et al., healing is enhanced if the portion of a body having an open wound is sealed inside a bag filled with hyperbaric oxygen at a precise low pressure. This pressure must be maintained within rigid bounds for the treatment to be effective. Typical treatment regimes call for application of oxygen for several hours a day for several weeks.
Many instruments are available for monitoring high pressure. However, in the aforementioned medical application, the upper end of the pressure range is on the order of 0.05 psi, a value so low that most conventional gauges are not capable of making the measurement. (A pressure of 0.05 psi will push water up a drinking straw only 1.4".) Furthermore, electronic pressure transducers may not be used around oxygen for fear of fire due to the possibility of spark production. Expensive mechanical pressure devices are also not desirable for medical applications where everything used in the treatment must either be disposed of as a biohazard or sterilized after each use. The indicator also should be readable across a room (about 10 feet), and it must be useable without on-site calibration.
U.S. Pat. No. 3,546,944 of R. Mack discloses an inexpensive device that adheres at two spaced locations on an inflatable bag used as a splint. In most of the disclosed embodiments, the Mack device breaks if the pressure exceeds a predetermined value on the order of 40 psi. However, the embodiment of FIG. 10 of the Mack patent has an elastic strip fastened between two spaced locations, and a scale attached at one location that provides an indication of pressure as the strip expands due to pressure in the bag. FIG. 11 shows measurements as low as 0.1 psi that appear to have been made with the elastic strip embodiment of Mack.
The non-linear measurements shown in FIG. 11 indicate that Mack's strip is in contact with the surface of his bag, where friction effects the accuracy of the reading. This friction makes it very unlikely that this device would perform well enough for the medical application at measurements below 0.1 psi. Furthermore, if Mack's device were made sufficiently small to minimize the effect of friction, it would be very difficult to read at any distance.